1. Field of the Invention
The present invention relates to an image forming apparatus including a belt member which is put on a stretching member with tension and can be endlessly moved.
2. Description of the Related Art
It is conventionally known in image forming apparatuses such as a copying machine and a printer for forming a color image to superimpose monochromatic toner images (e.g., yellow, magenta, cyan, black) of different colors by way of a belt member to form a full color image.
An intermediate transfer method, direct recording method and the like are known as a method of superimposing the toner images on the belt member. For example, in describing an image forming apparatus using four different color toners as an example, the image forming apparatus of the intermediate transfer method or direct transfer method may be a so-called 4-drum system or 1-drum system.
In further describing the image forming apparatus of the intermediate transfer method, in the 4-drum system, a plurality of image forming units for forming the toner image each has a photosensitive drum and forms a monochromatic toner image. The toner images are superimposed on the intermediate transferring belt to form a full color image. In the 1-drum system, the monochromatic toner image of the first color is first formed by a plurality of developers and one photosensitive member, and the toner image is transferred onto the intermediate transferring belt. Thereafter, the developer is sequentially changed, and the toner image of different color is similarly sequentially superimposed onto the intermediate transferring belt to form the full color image. In a case of the direct transfer method, the toner image is directly transferred and superimposed from the photosensitive drum 1 onto the recording material borne on the recording material bearing member instead of the intermediate transferring member.
In most of the image forming apparatuses of 1-drum system, a position detection mark (home position mark) is provided in a non-image region of the belt member, and a position detecting sensor (home position sensor) for reading the home position mark is arranged. This is because a so-called registration, or accurately aligning and superimposing the toner image of each color, is necessary in the 1-drum system. In other words, registration is performed by starting the image forming process of each color with the home position detection of the intermediate transferring belt as the reference.
In the 4-drum system, image aligning (registration) of each color such as in the 1-drum system is generally not performed. Normally, each image forming unit of each color forms a registration mark on the belt member before performing image formation. A registration sensor for detecting the registration mark is arranged in the image forming apparatus. The registration position shift amount of each color is obtained from the registration mark detection result of the registration sensor, and the registration position is corrected.
In the 4-drum system, the image forming unit of each color of yellow, magenta, cyan, and black is normally arranged in one row along the outer peripheral surface of the belt member. The belt member is generally put on rollers serving as a plurality of stretching members with tension. A drive roller out of the rollers drives the belt member and the belt member moves (travels) endlessly.
The belt member generally tends to have an uneven thickness in the longitudinal direction (traveling direction) due to manufacturing causes. Thus, speed variation occurs with one rotation of the belt member as one cycle even if the rotation speed of the drive roller for driving the belt member is controlled to a constant.
Since each image forming unit is arranged in one row along the outer periphery of the belt member as described above, the timings to start the image forming process of each image forming unit differ from each other according to the respective distance from the drive roller. Thus, image alignment of each color becomes very difficult when the speed variation caused by variation in the thickness of the belt member occurs as described above.
In the 4-drum system, a home position mark is provided on the belt member as a method of avoiding speed variation caused by variation in thickness. In other words, the thickness variation profile of the belt member is measured in advance in relation to the absolute position on the belt member determined from the home position mark. The image forming timing of each color and the rotation speed of the drive roller are controlled based on the thickness variation profile, so that the image alignment (registration) of each color is accurately performed.
The art for providing a reference mark on the belt member includes techniques disclosed in Japanese Patent Application Laid-Open No. 2000-132048 and Japanese Patent Application Laid-Open No. H05-150574.
Recently, upgrading the version of the image forming apparatus is being attempted for the purpose of further enhancing the image quality and of enhancing productivity with substantially the same cross-sectional configuration. Lowering the cost by changing the material and the like is also being attempted. Thus, difference in models tends to occur due to such changes.
When such change is effected, the change in the image forming process such as change in toner formulation and change in transfer property is often required. In components such as the intermediate transferring belt, the die is common, and the change is often effected by changing the material of the belt material, the method of processing the surface and the like. This is because the difficulty level of manufacturing the die is extremely high.
In this case, the belt member has exactly the same shape, and thus is very difficult to distinguish the difference based on outer appearance. As it is normally difficult to continuously maintain the transfer property until the intermediate transferring belt reaches the product life time of the image forming apparatus, the intermediate transferring belt is configured so as to be easily exchanged in the market. Troubles such as image defect occur if, for example, the intermediate transferring belt is arranged in a different model by mistake. In the worst case, the image forming apparatus may breakdown.
Japanese Patent Application Laid-Open No. 2001-005527 discloses a method of arranging a recognition display for indicating the transfer property in the intermediate transferring belt, arranging a detecting means for detecting the recognition display in the image forming apparatus main body, and controlling the transfer condition according to the detection result of the detecting means.
However, the conventional method of determining whether or not the belt member is suited to the image forming apparatus involves increase in cost and size of the apparatus.